"Bill Barr is acting more like the President’s personal attorney than the Attorney General of the United States. His job is to defend the rule of law and serve the American people, not shield the President from justice. He made something very clear today: he must resign," Harris wrote in her email to supporters.

"I’m running for president because the American people deserve truth and integrity from their elected leaders. That’s not what we’re getting right now. If you’re with me in this fight, I need you now," she added.

"William Barr has shown with actions in his handling of the Mueller Report’s release and with words in his testimony to Congress today that he’s put his political loyalty to Donald Trump before his duty to our country," Booker said in his email to supporters while also calling for Barr to resign.

The attorney general has emerged as a top target of Democratic presidential contenders after it was revealed Tuesday that Mueller expressed frustration to Barr about the attorney general's four-page summary of the Russia probe sent to Congress in late March.

Mueller said in a letter to Barr that the summary of the special counsel probe "did not fully capture the context, nature, and substance of this office’s work and conclusions."